Markets for conductive inks
While the membrane-switch industry is mature with much less growth than it once had, other applications for functional, conductive inks have been commercialized, and newer applications continue to be explored. Some of the other existing and new applications include disposable medical sensors such as EKG, EEG, and blood glucose sensors; electroluminescent panels; thin, flexible antennas; flexible, self-regulating heaters; bus bars for touchscreens; heater elements for glass windows; collector grids for solar panels; and plastic films molded into 3D shapes.
Disposable medical sensors represent one of the largest and fastest growing segments of the printed-electronics industry. These devices are printed much in the same way as membrane switches, with two or three separate layers of functional inks and graphic inks on the reverse side of the device. They use mixtures of silver and silver/silver-chloride inks that interact with an applied gel material to detect trace electrical signals on the surface of the skin produced by either the contraction of heart muscles (ECG or EKG sensor), or the firing of neuron networks in the brain (EEG sensor). While some sensors are printed using flatbed screen printing, more applications are being pushed to rotary screen or gravure-type printing processes as volumes increase and costs are reduced.
A word of caution when working with silver chloride inks: Silver chloride is sensitive to light and will react with most metals. A good manufacturing method is to use yellow-filtered light over work areas and to use non-metal flood bars and mesh when using these inks.
While most uses for disposable medical sensors remain clinical, there is a lot of activity within the industry to develop integrated medical sensors and devices for the consumer market that can be used to monitor heart rates during exercise and for home use by patients with heart problems.
Disposable blood-glucose sensors are a simple circuit of a conductive ink that has a small amount of a special enzyme on it. When a drop of blood is placed in contact with the enzyme, a complicated, three-stage chemical reaction takes place, leaving a chemical that can then be measured by the blood-glucose-sensing device to determine how much glucose is in the blood. These blood-glucose-sensing units can now be purchased off the shelf in pharmacies and other retail stores by people with diabetes or those who want to measure blood-glucose levels as part of an exercise or diet program.
Electroluminescent (EL) panels (Figure 2) are not a new technology, and it seems as if the technology has struggled for years to gain acceptance into large, sustainable applications. The devices are a thin, flexible light panel that activates with the application of AC current. They have been used in cell phones, as automotive dash-panel or back lighting, as wearable displays, and in point-of-purchase advertising displays. Construction of an EL panel is similar to a membrane switch, but involves different materials.
The vast majority of EL panels are made by screen printing. The printer first deposits a layer of specialty phosphor ink onto a polyester (PET) film layer that has a thin layer of indium tin oxide (ITO) deposited onto it. Next, a layer of dielectric ink is carefully printed onto the phosphor-ink layer. This dielectric ink acts as a capacitor to gather the electrical energy and fire it more uniformly into the phosphor layer. Finally, a back electrode of either silver or carbon ink is printed onto the dielectric ink. The conductive ink is also used to print conductive bus bars around the perimeter of the circuit to more evenly distribute electricity to the panel. EL-panel manufacturing continues to be a significant market segment, but it is somewhat threatened by recent developments of easier to print light panels made using complex phosphor materials that can operate on low DC current.
Thin, flexible, conductive antennas for cell phones and RFID applications (Figure 3) can be printed using conductive inks. With the high cost of silver and limitations of conductive, functional inks with respect to conductivity, the use of these inks is somewhat limited to certain segments of the antenna market. Up until about five years ago, some sources suggested that the market for conductive, functional inks for making printed antennas would be in excess of $100 million per year. Rapidly rising silver costs and limited conductivity pushed most of this technology over to stamped, thin metal foils and other methods of manufacturing. However, there are still several companies worldwide that continue to use silver inks for making flexible antennas.
Printed, flexible, self-regulating heaters (Figure 4) may be one of the areas of high growth in printed electronics over the next several years. Conventional heaters for applications such as automotive mirrors and seats require the use of a resistive heater (metal wires in the case of seats) that are regulated, temperature wise, by a thermocouple device. The thermocouple measures the heater temperature and sends an appropriate signal to increase or decrease electricity to the heater to maintain temperature.
One of the problems with these heaters is that if the thermocouple circuit were to fail, the device could have runaway heating. A printed, flexible heater uses a carbon-based Positive Thermal Coefficient (PTC) ink. When low voltage is applied to the heater, the PTC warms up until it reaches a temperature range of between 60-75°C. At this point, the polymer in the ink expands sharply, pulling the carbon particles further apart and effectively shutting of the flow of electricity through the heater. Once the heater cools slightly, the polymer shrinks and the device begins conducting once more. Manufacturing PTC heaters usually involves only two steps: printing silver-ink bus bars on the edges of the circuit and then overprinting a pattern using PTC ink.
Using conductive, functional inks for bus bars on touchscreens has been around for quite some time, but as these devices continue to evolve, there is a push to get printing technology that will allow for thinner ink traces. Current technology allows for traces to be printed as low as 100 μm, and the industry continues to push the envelope to get even thinner traces. There are some claims of lines as thin as 30 μm wide being printed using specialty meshes and tightly controlled printing processes.
Heater grids for glass windows and solar-panel collector grids both use a version of silver functional inks referred to as fired inks. These inks contain a fine-particle glass frit and a polymer base that has a low ash content when burned off. After printing and drying off the solvent, the substrate is put into a high-temperature oven and heated as high as 800°C. During this heating process, the polymer binder is burned off completely and the glass frit melts and causes the silver to sinter and form a solid layer of conductive metal.
This technology has been used by manufacturers of integrated automotive-glass heaters for many years. It has been more recently adopted for use in solar panels. Thin traces of silver-frit ink are printed over the length of the solar-cell face. These traces collect the electricity generated by the interaction of sunlight with the photovoltaic cell. A unique feature of the inks used in this application is that the ink has to be able to print a relatively thin trace but maintain thickness so that conductivity is maximized to allow the conductive grid to pull electricity away from the solar cell more efficiently.
The catch-22 is that when a conductive trace is printed at greater widths to minimize resistance along the length of the trace, the trace’s shadowing effect increases, blocking sunlight from reaching the solar cell. The inks must also be able to tunnel into the top layer of the semiconductor wafer used in the solar cell without breaching the layer completely. Frit inks for solar-panel manufacturing represent a very large market, and research continues to find new ways to make these inks work more efficiently with the cells.
Printing conductive inks onto flat substrates and then forming the substrate into 3D shapes (Figure 5) with heat and pressure, while maintaining the conductive ink traces, is a technology that has been demonstrated for quite some time. Recently, some applications, particularly automotive, have been driving the use of thermoformable, conductive-ink technology (Figure 6) to integrate electrical circuitry into dash panels, headliners, and other interior components of cars. Integrating circuitry onto the back side of molded plastic parts presents some opportunities for new design considerations, reducing manufacturing costs, and improving reliability. This technology represents a potentially very large opportunity within printed electronics.
Conductive, functional inks continue to provide good, value-added business for printers looking to expand into the growing printed-electronics market. With some knowledge of the chemistry behind these inks and an understanding of the technical requirements of the specific applications, the sky’s the limit.
Don Banfield, Conductive Compounds
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